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# The 15 que asking me the velocity at 4sIf we differentiate x we get velocity as 61 m/s but when we but value of t as 4 in eq of x we get 134 m and by further dividing it by 4 we get velocity as 33.5 m/s ....why there is a difference....plz help me

Tanishq Gupta
31 Points
3 months ago
Hello Preet,
The velocity that you have calculated by differenciating ‘x’ with respect to time is absolutely correct that is 61 m/s.
Now as mentioned in the question the particle is in uniform acceleration motion so therefore in the condition of acceleration motion we cannot apply the formula ‘velocity = displacement / time’ for that the velocity must be constant.
Instead in the condition of acceleration motion we have to apply the equation of motion.
Verification : x=2+5t+7t2
initial velocity (u) = 5 m/s (at t=0)
acceleration (at t=4) = 14 m/s2
by 1st equation of  motion : v=u+at
v=5+14(4)
v=61 m/s
Hence Proved.
Hope you find it useful.